In wheels, automotive wheels are generally made of steel or light alloys such as aluminum alloys and magnesium alloys.
Steel wheels are fabricated by pressing or roll forming. The formed wheels vary greatly from the intended dimensions. Especially, the roundness of the bead seat of the rim often deviates from the desired value. Furthermore, steel wheels are heavy and hence are not desirable, since all automotive parts tend to be reduced in weight.
In contrast, wheels made of light alloys are formed with stable dimensions. In addition, these wheels are much lighter than steel wheels. That is, the weights of light alloy wheels are one third those of steel wheels. However, the materials of light alloy wheels cost three to five times higher than the materials of steel wheels. In this way, light alloy wheels are very expensive.
In recent years, emphasis has been placed on energy saving. To achieve this saving, it is quite important to reduce the weights of automotive parts. Especially, unsprung members such as wheels have been required to be made lighter in weight to improve the fuel economy and maneuverability.
In these circumstances, a resin wheel which is satisfactorily lightweight, can be molded stably, and is economical to fabricate has been proposed recently. This resin wheel consists mainly of a fiber-reinforced plastic, i.e., a resin reinforced with short or long fibers. The resin wheel is lighter and molded with greater stability than the metallic wheel. In addition, resin wheels are adapted for mass production. Therefore, the manufacturing costs can be reduced. Further, the resin wheel is expected to be excellent in colorability and other design factors. Such a resin wheel is required to have the following characteristics.
(1) It is highly resistant to shock, for preventing destruction. PA1 (2) It creeps only a little, to prevent the tire from coming off the wheel. PA1 (3) It is highly resistant to heat generated by the brake drum. PA1 (4) It is excellent in uniformity. Since the wheel is one of the important parts for the automobile, if the molded wheel is nonuniform or has a defect, the wheel cannot be used. PA1 (5) It can be mass-produced.
The upper half of the prior art wheel is shown in FIG. 4. The wheel includes a rim 1 and a disk 2 provided with a hub bolt hole 3. Commercially available aluminum wheels are roughly classified in terms of construction into two major categories: (1) one-piece wheel that is fabricated by molding the rim 1 and the disk 2 integrally; and (2) two-piece wheel which is fabricated by molding the rim 1 and the disk 2 separately and then coupling them together by metal bolts. The two-piece wheel has the great advantage that various combinations of wheels can be offered by changing the design of the disk 2 while retaining the construction of the rim 1. However, the two-piece wheel must have overlapping portions, because the rim 1 and the disk 2 are joined together. Furthermore, the two-piece wheel is heavier than the one-piece wheel, because the overlapping portions are joined together with the metal bolts.
Of course, the light weight of the resin wheel is its prime selling point. To exploit this merit, any construction which increases the weight should be avoided as much as possible. Therefore, the present situation is that the prior art resin wheel takes the form of a one-piece wheel, whether it is fabricated by injection molding or by compression molding.
Where the prior art resin wheel is fabricated by compression molding, the fibers are damaged relatively slightly at the time of the molding. Therefore, the molded wheel is excellent in rigidity, strength, and other factors. In addition, the molded wheel has good heat resistance and good creep resistance, since the used resin consists mostly of a thermosetting resin. However, this molding method is unsuited for molding of complex shape such as a wheel whose wall thickness varies greatly from location to location. Furthermore, the productivity is low, because the molding cycle is long.
In contrast, the injection molding process is quite excellent in productivity and reduces the manufacturing costs. However, the strengths of products fabricated by injection molding are generally one third to one fifth of the strengths of products fabricated by compression molding. Specifically, when resins are kneaded or flow, the fibers are damaged or flow. Then, the fibers and the molecules are oriented, or a weld line is created at the point at which two masses of resin meet. As a result, the physical properties of the material of the molded product become nonuniform or deteriorate, leading to reductions in the strength. Especially, in the case of a wheel, weld lines often occur, partly because the disk is provided with numerous decorative holes to enhance the effect of cooling and for aesthetic reasons, and partly because bolt holes exist to join the drum of the body of the automobile. Because of frequent occurrence of weld lines, resin wheels have not yet been manufactured by injection molding in practice, nor resin wheels fabricated by injection molding have been put into the market.